This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Background: Pair housing of laboratory macaques is widely considered to lead to positive changes in wellbeing, yet the process of introduction is viewed as potentially stressful and risk-prone. Methods: Behavioral and physiological data were collected on eight adult male rhesus macaques before, during, and after the process of introduction, in order to measure the initial stress of introduction as well as long-term changes in wellbeing. Socially experienced subjects, all implanted with biotelemetry devices, were studied in five successive phases: baseline (singly housed), one day each of protected contact and full contact introduction, post-introduction (1-3 weeks after introduction), and settled pairs (e20 weeks after introduction). 176 h of behavioral data and 672 h of heart rate data were analyzed. Fecal cortisol was also measured for the baseline, post-introduction and settled pair phases. Results/Discussion: All introductions were successful and subjects showed no physiological or behavioral signs of stress, such as increased heart rate, abnormal behavior, or psychological indices of distress (depressive/anxiety-related behavior). Agonism was minimal throughout the introduction process and over subsequent months;only one wound was incurred over the course of the study. Levels of abnormal behaviors, psychological indices of distress, locomotion, inactivity, and affiliation showed improvements within several weeks after introduction;these changes were still present 5-9 months later for the latter two categories. Heart rates during introduction fell significantly in the settled pair phase, and also varied predictably with time of day. Fecal cortisol levels were lower in settled pairs than in single housing. The fact that reductions in abnormal behavior did not persist over long-term may have been confounded by increasing duration of time spent caged. Results of this study may be of practical use for designing and monitoring social introductions and suggest that managers should not dismiss feasibility of successful pairing of adult male rhesus macaques.